Adaptive modulation and coding (AMC) is used to meet the high demands on throughput in modern wireless systems. The idea of AMC is to change the modulation format and coding rate “on the fly” in order to adapt to a changing channel quality. This adaptation mechanism is typically supported by a control channel on which the currently used modulation and coding parameters are signaled. These control channels themselves are quite information heavy in a modern wireless system and would benefit from the use of AMC in order to conserve channel resources in favor of payload data. Then, in principle, the modulation and coding parameters of these control channels would need to be sent over a “control channel for the control channel”, and so on.
However, instead of explicitly signaling the AMC parameters, so-called blind decoding could be used. The basic idea of blind decoding is to, “blindly”, try to decode data collected from the channel by trying different combinations of modulation formats, channel codes and code block lengths. The set of valid combinations to try is agreed upon on beforehand. The blind decoding is supported by a Cyclic Redundancy Check (CRC), which is added to the information sequence. When the CRC matches after a decoding attempt, it is assumed that the correct modulation format, channel code and block length have been found, and that the data is uncorrupted. In some multi-user wireless systems, such as e.g. LTE, the CRC check is also used to pinpoint the intended recipient of the transmitted information, by using user specific CRC codes [1].
First, a short review of how blind decoding is implemented in LTE:
In LTE, the Physical Downlink Control Channel (PDCCH) supports different transmission formats for the Downlink Control Information (DCI), which are a priori unknown to the terminals (UEs). Each terminal will find its information by blindly decoding the incoming information by trying a set of possible formats. More precisely, the transmission of control information in LTE can be described as follows:
There are five DCI formats with different message sizes. Based on factors such as cell-coverage, the number of terminals in the cell and the scheduling granularity, one format is chosen as the downlink control information format by a transmitting node, such as an eNB. Prior to transmission, a terminal-specific CRC is appended to each control message. The attached CRC is used by each terminal to find the control information. After attaching the CRC, the control information bits are encoded with a rate-⅓ tail-biting convolutional code and the rate is matched to fit the amount of resources available for PDCCH transmission by using a circular buffer.
The mapping of PDCCHs to physical resource elements is subject to a certain structure which is based on so-called Control Channel Elements (CCE). Each CCE consists of 36 physical resource elements. Based on the instantaneous channel condition and the DCI format, the PDCCH for each terminal is mapped onto a set of CCEs. Since various aggregations of the CCEs may be used for the transmission of control information, the terminal needs to blindly detect the format of the PDCCHs by testing different CCE combinations. To reduce the complexity of the blind decoding process in LTE, the search space of each terminal which describes the set of CCEs that the terminal is supposed to monitor for possible control message is limited to 44 possibilities per frame [1].
When reviewing the prior art on ways for improving the blind decoding process, the prior art falls into two main categories, which are:                Algorithms for finding the location of the control information, and,        Algorithms for determining the correct code that is used by the encoder to encode the control data.        
In the first category, a solution to decrease the computational effort of the blind decoding for LTE is proposed in the prior art. The idea is to limit the number of combinations in which the control data blocks may be arranged and located in CCEs, hence reducing the search space. For example, a tree-based concatenation of CCEs is proposed in the prior art, where the largest-sized CCE aggregation is a concatenation of smaller-sized CCE aggregations. To find the correct aggregation of CCEs, a terminal starts with the set having the smallest-size of CCE aggregation and continues with the combinations of those smaller-size CCE aggregations to search for a larger size CCE aggregation and so forth.
As discussed above, a PDCCH transmission employs circular buffer based rate matching for a rate-⅓ tail-biting convolutional code. Due to repetition of coded bits and search space overlapping between different CCE aggregation levels, multiple aggregation levels may pass the CRC checking. In the prior art, a solution to decrease the detection errors is proposed. The idea therein is to modify the circular buffer, for instance by excluding at least one coded bit from the circular buffer, such that the control information can be decoded unambiguously by the users.
In the second category, a solution to decrease the number of blind decoding attempts for high-speed downlink packet access (HSDPA) is proposed [2]. The idea is to decode the received data partially for each parameter/location combination in the search space and early terminate the decoders that yield low quality metrics. The quality metric can be cumulative log-likelihood ratios for instance.
In [3], an algorithm to estimate the parameter of a convolutional code from noisy observations in a Binary-Symmetric Channel (BSC) is proposed. The algorithm is based on so-called Expectation Maximization (EM). The authors showed that the computations can be simplified by using the concept of log-likelihood ratio algebra [4].
The blind decoding type of AMC comes at the price of a decoding delay and a considerable energy consumption in the decoder on the receiver side. Given that the receiver is a mobile device with limited battery capacity, especially the latter is of great concern, and any reduction in the decoding complexity incurred by the blind decoding strategy would be valuable.